Recently, as a power source for equipment requiring a high performance of power characteristics such as the hybrid electric vehicle (HEV) or the pure electric vehicle (PEV), an alkaline battery, particularly a nickel-hydrogen battery has been used. Generally, as the hydrogen storage alloy used for a negative electrode of a nickel-hydrogen battery, a hydrogen storage alloy produced by substituting a part of an AB5 rare earth hydrogen storage alloy such as LaNi5 with an element such as aluminum (Al) and manganese (Mn) is used. Since the AB5-type hydrogen storage alloy contains aluminum (Al) or manganese (Mn) having a low melting point, it is known that in the alloy, a segregation phase such as an aluminum-rich phase or a manganese-rich phase is likely to be formed in a grain boundary or on the surface thereof.
When the charge-discharge cycle is repeated, by the expansion or contraction of the crystal lattice of a hydrogen storage alloy, within the crystal of a hydrogen storage alloy, a large internal stress is generated. By such a large internal stress, the hydrogen storage alloy is pulverized or by the dissolution of aluminum (Al) or manganese (Mn) out of a formed segregation phase, the intergranular corrosion of the hydrogen storage alloy is caused, which gives rise to problems with the corrosion resistance of the hydrogen storage alloy. A method for causing the hydrogen storage alloy to consist of a monophase without forming a segregation phase by subjecting the hydrogen storage alloy to a thermal treatment has been variously investigated, for example in JP-A-Sho 62-31947.
However, in the procedure proposed in JP-A-Sho 62-31947, there was the following drawback. That is, when the hydrogen storage alloy is caused to consist of a monophase by subjecting it to a thermal treatment, there is no segregation interface, so that the area of the alloy contacted with an alkali electrolyte liquid decreases, so that the initial activation performance of the battery is lowered. Therefore, there was also the problem that satisfactory charge-discharge properties and satisfactory cycle life properties for the application of the hybrid electric vehicle (HEV) and pure electric vehicle (PEV) requiring power characteristics much more beyond the related-art range, cannot be obtained.
Normally, a general hydrogen storage alloy has the above-described AB5-type structure or AB2-type structure. However, it is known that by combining the AB2-type structure and the AB5-type structure, the hydrogen storage alloy takes various crystal structures. Among them, a hydrogen storage alloy in a Ce2Ni7-type structure in which the AB2-type structure and the AB5-type structure are superimposed on each other with a cycle of two layers has been variously investigated, for example in JP-A-2002-164045. The hydrogen storage alloy in the Ce2Ni7-type structure has a crystal structure of a hexagonal 2H form and can improve cycle life characteristics.
However, there was the problem that the hydrogen storage alloy in the above-described Ce2Ni7-type structure proposed in JP-A-2002-164045 has unsatisfactory discharge characteristics (assist power) and lacks satisfactory performance for the application of high power much beyond the related-art range. Here, a hydrogen storage alloy consisting of a rare earth element, nickel and magnesium take various crystal structures in a combination of the AB2-type structure and the AB5-type structure and consists of besides the AB2-type structure and the AB5-type structure, the A2B7-type structure, the A5B19-type structure which are a metastable phase.
These component ratios are greatly vary depending on the stoichiometric ratio of the hydrogen storage alloy. For example, in a stoichiometric ratio region higher than a related-art region, the ratio of nickel is high, so that the melting point amplitude becomes large during the metal dissolution. Therefore, it is known that aluminum and magnesium segregate, so that aluminum forms a segregation phase of a AB5-type structure and magnesium forms a segregation phase of a AB2-type structure, which leads to degradation of alloy corrosion resistance. However, the battery properties resulting from the component ratio of the A2B7-type structure and A5B19-type structure, which are a metastable phase, were not clear.